LT3740 Datasheet, PDF (8/20 Pages) Linear Technology – Wide Operating Range, Valley Mode, No RSENSE™ Synchronous Step-Down Controller

LT3740
APPLICATIONS INFORMATION
Current Sensing Range
Inductor current is determined by measuring the voltage
across a sense resistance – either the on-resistance of
the bottom MOSFET or an external sensing resistor. The
maximum current sense threshold has three steps that are
selected by the RANGE pin. The current sense threshold
voltage without slope compensation is shown in Table 1.
This is the value for high duty cycle operation.
Table 1. Current Sensing Thresholds
RANGE PIN
CURRENT SENSING THRESHOLD
Ground
50mV
Open
80mV
V_IN
105mV
Slope Compensation
The LT3740 has a compensation slope to stabilize the
constant-frequency valley mode operation. The slope
compensation signal increases with the bottom gate duty
cycle, which results in a current sense threshold voltage
change with duty cycle as shown in the figure in Typical
Performance Characteristics. The three current limit levels
correspond to three compensation slopes.
The compensation slope needs to overcome the difference
between the up and down slope of the inductor current
to avoid sub-harmonic oscillation. Maximum compensa-
tion slope is required for high input voltages, where the
duty cycle is small. The compensation slope can only be
selected by the RANGE pin. In the case of insufficient
compensation slope, the inductor ripple current or the
sensing resistance needs to be reduced.
Reverse Current Limit
Because the LT3740 operates in forced continuous mode
when the feedback voltage is higher than 720mV, the
inductor current can go negative on occasion, such as
light load, shutting down with a slow SHDN signal, large
load step-down transient response, or the output voltage
being pulled up by some other power supply. The LT3740
has a reverse current comparator to limit the reverse
current. During the on-time of the bottom MOSFET, when
(V_SN+)–(V_SN–) reaches 40mV, the comparator is triggered
and turns off the bottom MOSFET.
When operated under light load, the inductor current goes
negative every cycle. The design of the inductor current
ripple and the sensing resistor need to ensure that the
reverse current comparator is not triggered during normal
operation.
Power MOSFET Selection
The LT3740 requires two external N-channel power
MOSFETs, one for the top switch and one for the bottom
switch. Important parameters for the power MOSFETs are
the breakdown voltage V_(BR)DSS, threshold voltage V_(GS)TH,
on-resistance R_DS(ON), reverse transfer capacitance C_RSS
and maximum current I_DS(MAX).
When the bottom MOSFET is used as the current sense
element, particular attention must be paid to the initial varia-
tion, the gate-source voltage effect and the temperature
characteristics of its on-resistance. MOSFET on-resistance
decreases as the gate-source voltage increases. The change
of BGDP voltage could affect the bottom MOSFET gate
voltage. Refer to the MOSFET datasheet for the MOSFET
on-resistance corresponding to certain gate voltage.
MOSFET on-resistance is typically specified with a
maximum value R_DS(ON) at 25°C. In this case, additional
margin is required to accommodate the rise in MOSFET
on-resistance with temperature:
R_DS(ON) = R_SENSE/ρ_T
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